The Effect of Zooplankton Grazing on Phyto-Plankton Biomass


  • Lethukuthula Mhlongo


Zooplankton grazing, phytoplankton biomass, herbivory, top-down control, eutrophic systems


An ex situ study investigating the effects of microzooplankton grazing on phytoplankton biomass was conducted at the Estuarine and Freshwater laboratory at the Nelson Mandela University (33°S, 25°E), from samples taken at North End Lake, in Port Elizabeth using the dilution technique. The study aimed at exploring the potential use of microzooplankton as a form of biocontrol of heavily polluted algal systems in relation to chemical controls. The North End Lake is a prime example of a hypereutrophic system that is prone to algal blooms consequently from industrial processes that increase the nutrient load in the system. Water samples were collected from the North-End Lake, in Port Elizabeth (33°S, 25°E), where three treatment groups were setup – a control treatment consisting of no grazers, a low-density grazer treatment and a high-density grazer treatment, at ratios of 1:4 and 1:1 of concentrated grazers against filtered water, respectively. A proportional relationship was predicted between grazer density and phytoplankton biomass; where increased grazing consequently of a higher density of microzooplankton will cause a significant decrease in the phytoplankton biomass. The results of the study found that at higher microzooplankton densities, phytoplankton biomass reduced significantly as compared to lower microzooplankton densities. Phytoplankton biomass decreased non-sequentially throughout the study. Initially, all three treatment groups had an average of 65µg/L, where phytoplankton biomass significantly dropped, to 10µg/L in the last week of the study for the control and low-density grazer treatments; the high-density grazer treatment recorded no biomass during the same period. Further statistical analyses outline that physicochemical parameters played a non-significant role in controlling phytoplankton biomass (p=0.0785 for temperature and, p=0.0652 dissolved oxygen). The microzooplankton community largely comprised of Copepoda and Rotifera species with a relatively lower number of Cladocera species.


Please cite this contribution as follows:

Mhlongo, K. (2021). The effect of zooplankton grazing on phyto-plankton biomass, "forsch!" - Studentisches Online-Journal der Universität Oldenburg, 1, 102-113.







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